Filter



March 10, 1964 D. 1... PFLAGER ETAL 3,124,649

' METHOD AND APPARATUS \FOR AUDIO REPRODUCTION Filed June 5, 1961 2 Sheets-Sheet 36 I6 R R R+ MIXER R L 20 22 30 I R 24 snzneo g M'XER -(R+L gig I W g; 34 L 38 FILTER L R+L l MIXER f 14 18 L I0 32 Fig. I

Fig. 2

INVENTORS. DENNIS L. PFLAGER By MAURICE R. JACKSON BUCKHORN, CHEATHAM 8 BLORE ATTORNEYS D. L. PFLAGER ETAL 3,124,649

March 10, 1964 METHOD AND APPARATUS FOR AUDIO REPRODUCTION 2 Sheets-Sheet 2 Filed June 5, 1961 INVENTORS. DEVNIS L. PFLAGER MAURICE R. JACKSON BUCKHORN, CHEATHAM 8 BLORE ATTORNEYS United States Patent 3,124,649 METHOD AND APPARATUS FOR AUDIO REPRODUCTION Dennis L. Pilager and Maurice R. Jackson, Portland, Oreg., assignors to Inter-Aural Research, Inc., Portland, Greg, a corporation of Oregon Filed June 5, 1961, Ser. No. 114,979 13 Claims. (Cl. 179-1) The subject matter of the present invention relates generally to audio reproduction and specifically includes apparatus and method for obtaining a realistic sound from one or more sources of electrical audio signals.

The invention applies to the recording of sound as well as to the play back of such recording and to the immediate reproduction of sound, such as in public address systems. In may be used to reproduce speech, music and any other audible sounds. The invention is particularly useful in the reproduction of music, which may employ any of a variety of sources of electrical audio signals, such as microphones, radio receivers, electrical musical instruments, phonograph records, magnetic tape, etc.

Briefly the process of the present invention employs an audio reproduction system having apparatus which utilizes a pair of electrical input signals in the audio range, which signals may be the same or different. The process separates a portion from each of the input signals, adds such separated portions together to form a mixture of signals containing portions of each of the pair of input signals, filters such signal portions which are added to form such mixture in order to obtain a mid-band range or frequencies, divides the filtered mixture of signals into two parts having similar frequency characteristics, re-

verses the phase of one of the parts of such mixture without changing the phase of the other of the parts, adds the phase reversed part of the mixture to the remaining input signal of one of the pair of input signals and the unchanged part to the remaining input signal of the other of the pair of input signals to form two separate output signals having mid-band ranges which are substantially reversed in phase and which contain the mixture of input signals, and converts each of the two output signals into sound.

The effect of the above described process is to provide the listener with orientation Within the space occupied by the sound, localization of such sound, and tonal balance of a quality heretofore unattainable. It differs from conventional monaural and stereophonic reproduction in that it provides an improved presence effect and allows tones in a selected mid-band range of frequencies to vary in phase relationship but in a controlled manner so that this variation does not exceed 180 degrees. This enables the conversion of monaural sound to provide an effect similar to binaural sound and an increase in the realism of reproduction of binaural sound.

Therefore, one object of the present invention is to provide an improved method and apparatus for audio reproduction.

Another object of the invention is to provide apparatus for improved audio reproduction which may be used to convert monaural sound into a sound signal which gives an effect similar to binaural sound or to increase the realism of binaural sound reproduction.

A further object of the present invention is to provide an audio reproduction system which controls the output of a plurality of electro-acoustic transducers so that they emit a selected mid-band range of frequencies which vary in phase relationship, but are controlled so that this variation cannot exceed 180 degrees. I

Still another object of the invention is to provide an electrical circuit for audio reproduction which employs 3,124,649 Patented Mar. 10, 1964 "ice at least a pair of audio signal transmission channels which are connected so that a portion of the input signal may be separated from each of such channels, added together and filtered to obtain a mixture of mid-band range of frequencies which contains such portions of input signals obtained from each of such pair of channels, and which are further connected so that part of this mixture of midband frequencies is substantially reversed in phase without affecting the phase of the other part of such mixture so that such phase reversed part may be added to the output of one of such channels and the other part of such mixture may be added to the output of the other of such pair of channels.

A still further object of the invention is to provide an audio reproduction system which includes a pair of sources of electrical sound signals, a pair of electroacoustic transducers for converting such signals to sound, a pair of supply circuits connected between said sources and said transducers for supplying different audio signals to each of said transducers, a first mixer device connected to each of such supply circuits for obtaining a portion of each of the input signals from said sources and for adding such portions together to form a mixture of signals, a filter device connected to such first mixer for separating a mid-band range of frequencies from such portions which are added to form such mixture, a phase inverter device employed to substantially reverse the phase of part of such filtered mixture of signals without substantially affecting the phase of another part of such mixture, and a second mixer device connected to such phase inverter device to add such phase-reversed part of such mixture to the output of one of such pair of supply circuits and add such unaffected part of such mixture to the other of such pair of supply circuits.

Additional objects and advantages of the present invention will become apparent after referring to the following detailed description of certain preferred embodiments of the present invention and to the attached drawings of which:

FIG. 1 is a block diagram of one embodiment of the present invention;

FIG. 2 is a block diagram of another embodiment of the present invention;

FIG. 3 is a schematic diagram of an electrical circuit in accordance with the embodiment of FIG. 1; and

FIG. 4 is a schematic diagram of an electrical circuit in accordance with the embodiment shown in FIG. 2.

One embodiment of the audio reproduction system of the present invention is shown in the block diagram of FIG. 1. This embodiment may include a pair of audio signal transmission channels 10 and 12 which may be positioned to the left and right, respectively, of the listener at their output ends as indicated by L and R Where, as described later, L and R refer to the mid-band frequencies only. The input ends of the channels 10 and 12 may be connected to two separate sources of electrical sound signals 14 and 16, respectively, such as electrical pre-amplifiers suitably connected to any of the sound sources described above. The output ends of the channels 10 and 12 may be connected to two separate electro-acoustic transducers 18 and 20, such as phonograph record or tape recording heads, ear phones or loudspeakers suitably connected to conventional power amplifiers. A suitable mixer device 22, such as conventional mixing transformers, mixing tubes, or combining networks, may be connected to the input ends of the channels 10 and 12 and a suitable filter 24 may be connected to the output of the mixer 22. This filter 24 may be any conventional band-pass filter which passes a mid-band range of frequencies between 400 and 4000 c.p.s. with some attenuation. A variable mid-band gain control in the form of a potentiometer 26 may be connected between the filter back into channel through a fourth a 24 and ground with the sliding arm contact of such potentiometer connected to a phase inversion device 35). The phase inversion device 30 may be any conventional device which substantially reverses the phase of an electrical signal transmitted therethrough, such as a split-load phase inverter. The output of the phase inverter 30 which supplies the phase reversed signal is connected to the channel 10 through a second mixer device 32. The other output of the phase inverter 30 which supplies the iii-phase signal is connected to the channel 12, through a mid-band balance control potentiometer 34 whose sliding arm contact is connected to ground, by a third mixer device 36.

As seen in FIG. 1, a switch 38 determines whether the system will be operated from a monaural source or a stereophonic source. When this switch is set in the position shown, a portion of the input signal supplied to the channels 10 and 12 by sources 14 and 16, respectively, is separated from each of such channels and supplied to the mixer 22 where the portion from the left channel 10 is added to the portion from the right channel 12. This mixture of input signals including the mid-band frequencies L and R may then be passed through the bandpass filter 24, which transmits only a mid-band range of frequencies between 400 and 4000 c.p.s. approximately. This mixture of mid-band frequencies (R-l-L) is then transmitted through the gain control potentiometer 26 to the phase inverter 30 where a part of this mixture is substantially reversed in phase from the other part. The expression (R-i-L) is employed since in the specific embodiment of the present application there is a phase inversion in the mixer 22. It should be understood that there is some phase shift in the filter 24 so that all frequencies of this phase reversed part of such mixture will not be 180degrees out of phase with the input signal. Assuming the part of mixture of mid-band frequencies supplied to the mixer 32 from the phase inverter 30 is 180 degrees out of phase with the same mid-band frequencies of the input signal channel 10, the amplitude of such part of such may be adjusted by the potentiometer until it is suflicient to cancel and override such channel tounity, as indicated by 2(R|L), so that the output from the mixer 32 will be 2R+L. Also, assuming that the in-pha se part of the mixture of mid-band frequencies supplied from the phase inverter 30 to the mixer 36 is of the same frequency and amplitude as the mid-band frequencies of channel 12, as indicated by R-i-L, the output from such mixer 36 will be equal to the -(2R+L) in this mid-band region. This means that the mid-band signal supplied to transducer 18 is substantially 180 out of phase with the mid-band signal applied to transducer 20. Of course, the band-pass filter 24 could be formed in two separate sections and moved from the position shown until they are connected between mixer 22 and the input ends of lines 10 and 12, respectively. Also, band rejection filters could be connected in channels 10 and 12 between their input and output ends to absorb the midband rang'e between 400 and 4000 c.p.s., so that the amplitude of the phase reversed signal added by mixer 32 need not be sufiicient to cancel and override the input signal from channel 10.

Another embodiment of the audio reproduction system of the present invention is shown in the block diagram of FIG. 2. This embodiment is similar to that shown in FIG. 1 except the band-pass filter 24 has been replaced by a pair of band-pass filters 40 and 42 of similar characteristic's which are connected between mixer 22 and sources 14 and 16, respectively. In addition, a second phase inverter device 44 is connected at its input 'end between band-pass filter 40 and mixer 22 so that part of the mid-band range of frequencies transmitted through filter 40 is substantially reversed in phase by phase inverter 44 and amplified to unity gain over the input signal L in channel 10, as indicated by 2L, before being added mixer device 46. Since the output from mixer 46 contains a mid-band range of frequencies substantially reversed in phase from the input signal of channel 10, as indicated by L, the setting of balance potentiometer 34may be changed so that the amplitude of the phase inverted part, R-l-L, of the midband mixture obtained from split-load phase inverter 30 has the same amplitude as the unaffected part, (R+L), of such mixture from such inverter. This is possible because the phase reversed part of such mixture need not override the mid-band frequencies of channel 10 to which it is added inorder to provide an output R+2L from mixer 32 which is substantially 180 phase shifted from the output (ZR-i-L) of mixer 36. Of course, band rejection filters could be inserted in channels 10 and 12 after the take-off for the band-pass filters 40 and 42 so that no mid-band range L is supplied to mixer 46 and the phase reversed mid-band from phase inverter 44 need not be amplified until it cancels and overrides such midband formerly in channel 10 at this point. Obviously, the mid-band output from mixer 36 would then be (R+L), rather than (2R+L), because channel 12 no longer transmits mid-band R to such mixer.

It should be noted that the mid-band range of frequencies supplied to transducers 18 and 20 in the embodiments of FIGS. 1 and 2 will not differ by 180 degrees at all frequencies thereof, but that in actual practice some of the frequencies in this filtered range of midband frequencies differ by less than the desired 180 degrees phase difference. Also, the setting of mid-gain potentiometer 26 should be set for best performance so that amplification of the mid-band mixture transmitted by phase inverter 30 is approximately 18 decibels above the audio signal in channels 10 and 12 at 800 c.p.s. and then declines in continuous manner with increasing frequency, at a rate of about 6 db per octave until unity gain relative to line is reached. In addition, when the selector switch 38 is moved to the monaural position it is obvious that much of the apparatus shown can be eliminated,- since only one of the sources 14 and 16 is supplying audio signals to the reproduction system.

FIG. 3 shows a schematic diagram of an electrical circuit which may be used for the audio reproduction system of FIG. 1. Each source of electrical sound signals 14 and 16 is connected through channels 10 and 12 to electroacoustic transducers 18 and 20. A portion of the input signal supplied by each source 14 and 16 is separated from channels 10 and 12 through a pair of voltage divider resistors 48 and 50 connected between the input end of channel 10 and ground and voltage divider resistors 52 and 54 connected between the input end of channel 12 and ground. A pair of RC band-pass filters arranged as unbalanced pi sections are connected with their input leads between the resistors of voltage dividers 48, 50 and 52, 54. These band-pass filter sections may each consist of an input capacitor 56 and 58 connected to ground, an output resistor 60 and 62 connected to ground and another capacitor 64 and 66 connected between said input capacitor and said output resistor. Each of these band-pass filter sections taken together corresponds to the band-pass filter 24 of FIG. 1. The outputs of the bandpass filters are connected to a mixer circuit including a pair of vacuum tube triod'es 68 and 76 connected in pushpull and having common cathode bias resistor 72 whose bias voltage is supplied by an electrolytic capacitor 74 connected in parallel with such cathode bias resistor so as to have the polarity indicated with respect to ground. The tubess and 76 have their anodes connected to a common source of D. C. plate voltage 76 through load resistors 78 and 80, respectively, so that they are connected as amplifiers with the input signal applied through the band-pass filter sections to the grids of such tubes. The filtered output signals, which appear on load resistors 78 and 80 reversed in phase, are added together through a pair of combining resistors 82 and 84 connected in series between such load resistors and the anodes of vacuum tubes 68 and 70. This mixer circuit corresponds to the mixer device 22 of FIG. 1 and supplies a mixture of mid-band frequencies from the right channel 12 and the left channel to the common connection of combining resistors 82 and 84, which have been reversed in phase by vacuum tubes 68 and 70 as indicated by the expression -(R+L).

The combining resistors 82 and 84 are connected at their common lead through a coupling capacitor 86 to a conventional split-load phase inverter, using a single Vacuum tube triode 88 of a high mu type. A potentiometer 90 serves as a mid-band gain control by having one end connected to the coupling capacitor 86 and its movable contact connected to the grid of tube 88. The anode of the vacuum tube 88 is connected to a source of DC. plate voltage 92 through a load resistor 94, while a cathode of such tube is connected to the lower end of the potentiometer 90 through voltage divider resistors 96 and 98 and a grid leak resistor 100. This potentiometer 90 and phase inverter circuit including the tube 88 correspond to the mid-band gain control 26 and the phase inverter device 30 of FIG. 1. Therefore, the output from the anode of tube 88 through a coupling capacitor 102 is reversed in phase as compared to the input signal supplied to the grid of such tube, as indicated by R+L, so that it is in phase with the input voltage R in channel 12 with which it is to be combined. The output from the cathode of tube 88 through coupling capacitor 104 has not been affected by the phase inverter, so that it remains in phase with the input to the grid of such tube as indicated by the quantity 2(R+L). The relative amplitude of the signals supplied through coupling capacitors 102 and 104 is determined by the setting of potentiometer 106 connected between coupling capacitor 162 and ground, which corresponds to the mid-band balance control 34 of FIG. 1. It should be noted that this balancing potentiometer could be connected instead as resistor 108 after coupling capactor 104. Since the signal supplied to capacitor 104 is reversed in phase from the input voltage L in channel 10 to which it is combined, the amplitude of this mid-band signal must be suificient to cancel and override the mid-band frequencies of such input voltage L.

The mixer circuits of FIG. 3 which correspond to the mixer devices 32 and 36 of FIG. 1 may be similar to the first mixer circuit containing vacuum tubes 68 and 70. Each of these mixer circuits consists of a pair of triodes 110, 112 and 114, 116 which are connected in push-pull having common cathode bias resistors 118 and 1211, respectively. These cathode resistors 118 and 120 are connected to ground in parallel with electrolytic capacitors 122 and 124, respectively. The anodes of each of the triodes forming a pair of mixer tubes 110, 112 and 114, 116 are connected to a common source of DC. plate voltage 126 and 128 through load resistors 130, 132, 134 and 136, respectively. Each of the grids of tubes 112 and 116 is connected to line through a pair of voltage divider resistors 138, 140 and 142, 144, respectively. The output from each of these mixer circuits is added through a pair of combining resistors 146, 148 and 150, 152 connected to the anodes of tubes 118, 112 and 114, 116, respectively. This output signal, indicated by 2R+L for channel 10 and (2R+L) for channel 12, is then transmitted through coupling capacitors 154 and 156 to transducers 18 and 20 connected across output resistors 158 and 160, respectively, where the electrical audio signal is converted into sound.

A schematic diagram of an electrical circuit which may be used for the audio reproduction system of FIG. 2 is shown in FIG. 4. This circuit is very similar to the embodirnent shown in FIG. 3, so that only the differences from that circuit will be hereafter described. The bandpass filter sections 40 and 42 of FIG. 2 are formed by the pair of RC filter networks including elements 56, 6t), 64 and elements 58, 62, 66, respectively, in FIG. 4, while the phase inversion device 44 is represented by vacuum tube 68. Since the output from the plate of tube 68 is 180 degrees out of phase with the input signal applied to the grid of such tube, it is this phase inverted signal which is supplied through coupling capacitor 162 to the grid of a triode vacuum tube 164. This tube 164 is connected as a cathode follower amplifier by a load resistor in the form of a pair of voltage divider resistors 166 and 168 connected between the cathode of such tube and ground. The coupling capacitor 162 is connected at one end between the combining resistor 82 and the anode of tube 68 and at the other end thereof to the grid of tube 164. A grid leak resistor 170 connects the grid of tube 164 between voltage divider resistors 166 and 168. The output signal supplied from the cathode of triode 164 is in phase with the input signal supplied to the grid of such triode, so that it remains reversed in phase as compared to the line 10. This phase inverted signal may be amplified to unity gain with respect line, as indicated by the expression -2L. Therefore, when this amplified phase inverted signal is added to the input signal in the left channel 16 by a pair of combining resistors 172 and 174 connected to source 14 and to the cathode of tube 164 through coupling capacitor 176, it cancels and overrides the line signal in the midband range of frequencies transmitted by the band-pass filter, as indicated by the expression -L, at the output of such combining network. This mixer circuit including tube 164 and combining resistors 1'72, 174 corresponds to the mixer device 46 of FIG. 2.

The above-described additional phase inversion and mixer circuits eliminate the necessity for the mixture of mid-band frequencies transmitted through coupling capacitor 104 to be of sufficient amplitude to cancel and override the line voltage in channel 16 because the midband range of frequencies -L supplied by such channel to tube 112 is in phase with those in such mixture (R+L) supplied to tube 118. It should be noted that a compensation resistor 178 has been connected to the input of line 12 and ground between source 16 and resistor 52. This compensation resistor 178 functions to balance the input impedance of channel 12 with that of channel 10 due to the added impedance of the mixer circuit including tube 164.

An example of typical values of circuit elements used in the embodiment of FIG. 4 is given below when triodes 68 and 70 are provided in a dual tube of the 12AX7/EOC83 type and triodes 88 and 164 are incorporated in a similar tube, while mixing triodes 110, 112 and 114, 116 are incorporated in a pair of dual tubes of the l2AU7/ECC82 type. Resistors 48 and 52 are 470 kilohms, resistors 50 and 54 are 100 kilohms, and resistor 178 is 27 kilohms. Capacitors 56, 58, 64 and 66 are each .001 microfarad while resistors 60 and 62 are 100 kilohms. Resistor 72 is 1.5 kilohms and capacitor 74 is 10 microfarads with a breakdown voltage of 50 volts. Resistors 78 and 80 are 100 kilohms, resistors 82 and 84 are 1.2 megohms while capacitor 162 is .002 microfarad and capacitor 86 is .0068 microfarad. Resistor 170 is 22 kilohms and resistor 100 is 10 kilohms. Resistors 96 and 168 are each 1 kilohm, and resistors 98 and 166 are each 27 kilohms. Capacitor 176 is .02 microfarad, resistor 172 is 22 kilohms, and resistor 174 is 20 kilohms. Resistor 94 is 100 kilohms and potentiometer is 50 kilohms. Capacitors 102 and 104 are both .02 microfarad, potentiometer 106 is kilohms, and resistor 108 is 100 kilohms. Resistors 118 and 120 are 1.8 kilohms while capacitors 122 and 124 are each 10 microfarads with a breakdown voltage of 50 volts. Resistors 130, 132, 134 and 136 are all 100 kilohms, resistors 148, and 152 are each 10 kilohms, and resistor 146 is 12 kiiohms. Resistors 140, 142 and 144 are all 100 kilohms, and resistor 138 is 47 kilohms. Capacitors 154 and 156 are both .068 microfarad, and resistors 158 and 160 are each 100 kilohms.

It is not intended to limit the scope of the present invention by the preceding detailed description of certain preferred embodiments thereof, since details of these embodiments may be varied in an obvious manner by one having ordinary skill in the art. Therefore, the scope of the present invention should be determined only by the following claims.

We claim:

1. An audio reproduction system comprising:

a plurality of electrical circuits each adapted to be connected at its output end to a separate electroacoustic transducer for supplying different audio out put signals to said transducers,

means to apply audio input signals to the input ends of said circuits,

means for subtracting a portion from each of said input signals, and for separating a mid-band range of frequencies from each of the subtracted input signal portions for combining at least two of said subtracted input signal portions from two different circuits of said plurality of circuits to obtain a mixture of the mid-band ranges of said two input signal portions, and adding a portion of this mixture of mid-band ranges to the remaining signal in each of said two circuits, and

means for substantially reversing the phase of one of said portions of said mixture of mid-band ranges before it is added to said remaining signal and converted into sound, so that the sound output from said transducers includes a mid-band range of frequencies emitted from one of said transducers which is substantially reversed in phase from that emitted by another of said transducers.

2. An audio reproduction system comprising:

a plurality of electrical circuits each having an input end and an output end with each of said circuits adapted to be connected to a seperate 'electro-acoustic transducer at said output end for supplying different audio output signals to each of said transducers,

means to apply audio input signals to said input ends of said circuits,

means for separating a portion of the input signal applied to said input end of each of said circuits and for filtering a mid-band range of frequencies from said separated portions of input signal,

means for mixing at least two of said separated input signal portions from two different circuits of said plurality of circuits to obtain a mixture of the midband ranges of said two input signal portions, and adding part of this mixture to the remaining signal in each of said two circuits at said output end, and

means for substantially reversing the phase of one of said parts of said mixture of mid-band ranges before it is added to said remaining signal and converted into sound, so that the sound emitted from said transducers includes a mid-band range of frequencies emitted from one of said transducers which is substantially reversed in phase from that emitted by another of said transducers.

3. An audio reproduction system comprising:

a plurality of electroacoustic transducers for converting electrical audio signals to sound,

a plurality of electrical supply circuits with each of said circuits connected at their output ends to at least one of said transducers for supplying different audio output signals to each of said transducers,

means to apply audio input signals to the input ends of said circuits,

means for separating a portion of the input audio signal applied to each of said circuits and for band pass filtering a mid-band range of frequencies from said separated portions of input signal,

means for mixing two of said separated portions of input signal from two different supply circuits and adding part of said mixture before it is added to said 8. remaining signal to the remaining signal in each of said two circuits, and

means for amplifying said mixture so that said parts have an amplitude which is greater than that of the mid-band of the remaining input signal of said two circuits to which said parts are added, and for substantially reversing the phase of one of said parts of said mixture before it is added to one of said two circuits so that the sound output from said transducers includes a mid-band range of frequencies emitted from one of said transducers which is substantially reversed in phase from that emitted by another of said transducers.

4. An audio reproduction system comprising:

a plurality of loudspeakers for converting electrical audio signals to sound,

a plurality of electrical supply circuits with each of said circuits connected at its output end to at least one of said speakers for supplying different audio output signals to each of said speakers,

means for applying audio input signals to the input ends of said circuits,

means for separating a portion of the input audio signal applied to each said circuits and for band pass filtering a mid-band range of frequencies between 400 and 4000 c.p.s. from said separated portions input signal,

means for substantially reversing the phase of part of one of said portions of separated input signal and for adding it back to a first circuit from which it was separated after said filtering,

means for mixing two of said separated portions of input signal from two different circuits including said first circuit and adding part of said mixture to each of said two circuits, and

means for substantially reversing the phase of one of said parts of said mixture before it is added to said first circuit so that the sound output from said speakers includes a mid-band range of frequencies emitted from one of said speakers which is substantially reversed in phase from that emitted by another of said speakers.

5. An electrical circuit for audio reproduction comprising:

a first audio signal transmission channel having an input end and an output end,

a second audio signal transmission channel having an input end and an output end,

means to apply audio input signals to said input ends of said channels,

first mixer means connected to said input ends of said first and second channels for obtaining a portion of the input signal from each of said channels and for adding said portions to form a mixture of signals from said first and second channels,

filter means connected to said first mixer for separating a mid-band range of frequencies from said portions of said input signals which are added by said first mixer,

phase inversion means connected to said first mixer and said filter for substantially reversing the phase of one part of the filtered mixture of mid-band signals produced by said first mixer and said filter, and

second mixer means connected between said phase inversion means and said output ends of said first and second channels for adding said phase reversed part of said filtered mixture to said first channel and for adding the in-phase part of said filtered mixture to said second channel.

6. An electrical circuit for an audio reproduction systern comprising:

a first audio signal transmission channel having an input end and an output end,

a second audio signal transmission channel having an input end and an output end,

first mixer means connected to said input ends of said first and second channels for obtaining a portion of the input signal from each of said channels and for adding said portions together to form a mixture of signals from said first and second channels,

filter means connected to said first mixer for separating a mid-band range of frequencies from said portions of said input signals which are added by said first mixer,

first phase inversion means connected to said first mixer and said filter for substantially reversing the phase of one part of the filtered mixture of midband signals produced by said first mixer and said filter,

second mixer means connected between said first phase inverter and said output ends of said first and second channels for adding said phase reversed part of said filtered mixture to said first channel and for adding the in-phase part of said filtered mixture to said second channel,

second phase inversion means connected to said first channel and said filter to obtain a portion of the input signal of said first channel which is filtered by said filter and for substantially reversing the phase of said portion of the input signal so obtained, and

third mixer means connected to said second phase i11- verter for adding said phased reversed and filtered signal portion back into said first channel.

7. An audio reproduction system comprising:

a pair of sources of electrical sound signals,

a pair of loudspeakers,

a pair of supply circuits connected between said sources and said loudspeakers for supplying difierent audio signals to each of said loudspeakers at the output ends of said circuits,

a first mixer device connected to said sources at the input ends of said pair of supply circuits for obtaining a portion of each of the input audio signals of said supply circuits and for adding said portions together to form a mixture of signals from each of said sources,

band pass filter network connected to said first mixer for separating a mid-band range of frequencies from said portions of said input signals which are added by said first mixer,

a split-load phase inverter circuit connected to said first mixer and said band pass filter for dividing said mixture of signals into two parts and for substantially reversing the phase of one of said parts without changing the phase of the other of said parts with respect to said input signals,

means for amplifying said portions of said input signals which are added by said first mixer and for varying the amplitude of said parts of said mixture so that said phase reversed part may cancel and override the mid-band of the remaining portion of said input signal in one of said supply circuits,

a second mixer device connected from said phase inverter to the output end of one of said supply circuits for adding said phase reversed part of said mixture to the remaining portion of said input signal in said one supply circuit, and

a third mixer device connected from said phase inverter to the output end of the other of said supply circuits for adding said other part of said mixture to the remaining portion of said input signal in said other supply circuit.

. An audio reproduction system comprising:

pair of sources of electrical sound signals,

pair of loudspeakers,

pair of supply circuits connected between said sources and said loudspeakers for supplying different audio signals to each of said loudspeakers at the output ends of said circuits,

a first mixer device connected to said sources at the input ends of said pair of supply circuits for obtaining a portion of each of the input audio signals of said supply circuits and for adding said portions together to form a mixture of signals from each of said sources,

band pass filter network connected to said first mixer for separating a mid-band range of frequencies from said portions of said input signals which are added by said first mixer,

a split-load phase inverter connected to said first mixer and said band pass filter for dividing said mixture of signals into two parts of combined mid-band and for substantially reversing the phase of one of said parts without changing the phase of the other of said parts with respect to said input signals of said supply circuits,

a second mixer device connected from said phase inverter to the output end of one of said supply circuits for adding said phase reversed part of said mixture to the remaining portion of said input signal in said one supply circuit,

a third mixer device connected from said phase inverter to the output end of the other of said supply circuits for adding said other part of said mixture to the remaining portion of said input signal in said other supply circuit,

a second phase inverter means connected to the input end of said one supply circuit to which said phase reversed part of said mixture is added through said filter network in order to obtain a portion of said input signal of said one circuit which has been filtered to separate said mid-band range, to substantially reverse the phase of said portion so obtained by said second phase inverter and to amplify said portion until it has a greater amplitude than said midband range of: the remaining portion of said input sigial to which it is added in said one supply circuit, an

a fourth mixer device connected to said second phase inverter means and to said one supply circuit before said second mixer device in order to add said phase inverter signal from said second phase inverter to said one supply circuit.

9. A method of audio reproduction comprising:

producing a pair of audio input signals, subtracting a portion from each of said pair of input signals,

separating a mid-band range of frequencies from each of lthe subtracted portions of said pair of input signa s,

mixing said subtracted portions to form a mixture of mid-band frequencies,

reversing the phase of one portion of said mixture without changing the phase of another portion of said mixture,

adding said phase reversed portion of said mixture to the remainder of one of said input signals and adding said unchanged portion of said mixture to the remainder of the other of .said input signals to produce a pair of audio output signals, and

converting each of said output signals into sound at different separated positions.

10. A method of audio reproduction comprising:

producing a pair of electrical input signals in the audio range, separating a portion from each of said pair of input signals,

adding said separated portions together to form a mixture without changing the phase of the other of said pair of input signals,

filtering said signal portions which are added to form said mixture in order to obtain a mid-band range of frequencies,

dividing said filtered mixture of signals into two parts having similiar frequency characteristics,

reversing the phase of one of said parts of said mixture without changing the phase of the other of said parts,

adding said phase reversed part of said mixture to the remaining input signal of one of said pair of input signals and said unchanged part to the remaining input signal of the other of said pair of input signals to form two separate output signals having mid-band ranges which are substantially reversed in phase and which contain said mixture of input signals, and

converting each of said two output signals into sound.

11. A method of audio reproduction comprising:

producing a pair of electrical input signals in the audio range,

obtaining part of the signal from one of said input signals, filtering said input signal part to provide a mid-band range of frequencies,

inverting the phase of said part so that it is reversed with respect to said one input signal,

adding said filtered and phase inverted part to the remainder of said one input signal,

separating a portion from each of said pair of input signals,

adding said separated portions together to form a mixture of signals containing portions of each of said pair of input signals,

filteringsaid signal portions Which are added to form said mixture in order to obtain a mid-band range of frequencies,

dividing said filtered mixture of signals into two parts having similar frequency characteristics,

reversing the phase of one of said parts of said mixture without changing the phase of the other of said parts,

adding said phase reversed part of said mixture to the remaining input signal of one of said pair of input signals and said unchanged part to the remaining input signal of the other of said pair of input signals to form two separate output signals having mid-band ranges which are substantially reversed in phase and which contain said mixture of inpupt signals, and

converting each of said two output signals into sound.

12. An audio reproduction system comprising:

a plurality of electrical circuits each having input and output terminals;

means for applying audio input signals to said input terminals of said circuits;

means for mixing a portion of said input signals together to obtain a mixture signal containing midband audio frequencies in the range of frequencies between 400 and 4000 cycles per second;

means for substantially reversing the phase of a portion of said mixture signal and transmitting the phase reversed portion of said mixture signal to the output terminal of one of said circuits; and

means for transmitting another portion of said mixture to the output terminal of another of said circuits so that the output signals of said one circuit and said other circuit are substantially reversed in phase at mid-band frequencies within said range of frequencies.

13. A method of audio reproduction, comprising:

producing a plurality of audio input signals;

mixing a portion of said input signals together to obtain a mixture signal containing mid-band audio frequencies in the range of frequencies between 400 and 4000 cycles per second;

inverting a portion of said mixture signal to substantially reverse the phase of said mid-band frequencies;

transmitting the phase reversed portion of said mixture signal to a first electro-acoustic transducer; and

transmitting another portion of said mixture signal to a second electro-acoustic transducer so that the output signals received by said first and second transducers are susbtantially reversed in phase at mid-band frequencies within said range of frequencies.

References Cited in the file of this patent UNITED STATES PATENTS 2,536,664 Sinnett et al. Jan. 2, 1951 2,845,491 Bertram July 29, 1958 FOREIGN PATENTS 744,831 Germany Nov. 25, 1943 817,773 Great Britain Aug. 6, 1959 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. .5 l24 649 March 10, 1964 Dennis L Pflager et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 7 line "Z5. to column 8', line 1 strike out "before it is added to said remaining signal" and insert the same after "mixture" in line 3, column 8; column l0 line 68, for "without changing the phase of the other" read of signals containing portions of each Signed and sealed this 14th day of July 1964.

(SEAL) Attest:

ESTON G. JOHNSON EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

9. A METHOD OF AUDIO REPRODUCTION COMPRISING: PRODUCTING A PAIR OF AUDIO INPUT SIGNALS, SUBTRACTING A PORTION FROM EACH OF SAID PAIR OF INPUT SIGNALS, SEPARATING A MID-BAND RANGE OF FREQUENCIES FROM EACH OF THE SUBTRACTED PORTIONS OF SAID PAIR OF INPUT SIGNALS, MIXING SAID SUBTRACTED PORTIONS TO FORM A MIXTURE OF MID-BAND FREQUENCIES, REVERSING THE PHASE OF ONE PORTION OF SAID MIXTURE WITHOUT CHANGING THE PHASE OF ANOTHER PORTION OF SAID MIXTURE, 